Ethernet controllers are devices or modules inside a larger device that manages the communications between the digital processing of a system and an Ethernet interface. It receives and transmits data to and from the local processing bus connection, adapts this flow to comply with the Ethernet standard (IEEE 802.3), and sends or receives the same data to the Ethernet bus.

 

The Ethernet bus connection can support 10,100 or 1000Mbits/s (10/100/1000Base-T respectively) serial communications on a shared physical medium. The Ethernet controllers that support these standards integrate the physical layer and data link layer circuitry – passing differential data to an isolating transformer for connection typically by an RJ45 connector to the Ethernet cable. Recently 10Gbit/S controllers have become available that require a separate active transceiver modules (PHY) to amplify and power the signal for either copper or optical fiber interfaces. The connection between the Ethernet controllers and the physical medium interface is called the medium dependent interface (MDI).

 

The local bus can be a number of formats. Some systems do not need high-speed communications, and the local bus interface is a slow serial connection (SPI or I2C). These controllers tend only to support 10Mbit/s (10Base-T) packet transmission speeds, but the actual data rate may be much lower. Other Ethernet controllers support a high-speed bus to cater for high data rates provided for by 100/1000Base-T. This utilizes high-speed local buses to transfer data in between itself and the microprocessor system. This is a parallel bus called the medium independent interface (MII), the reduced MII (RMII), or the Gbit/s supporting GMII or RGMII alternatives. 10Gbit/S Ethernet controllers have a very high-speed bus (XGMII) made up of two 32bit data buses, but the ten attached unit interface (XAUI) bus has largely replaced this. Some Ethernet controllers have an integrated PCI interface for direct connection to the processing systems PCI bus, incorporating direct memory access (DMA) for moving data to and from system memory.

 

Some controllers support multiple Ethernet connections and can be used as switches. To comply with the Ethernet standards, they typically include automatic polarity detection, collision detection, full and half duplex Ethernet modes, automatic retransmission, cyclic redundancy generation (CRC) and packet filtering. They usually incorporate data buffers to store one or more packets of data in both receive and transmit paths.